Almost all conventional mechanisms of two-shaft hinges which create sliding friction torque, click torque, or stress on the rotational shaft involve pressing one or more cam sets with a spring to create a sliding friction torque, click torque or stress (Japanese Patent Publication No. 2002-206520).
FIG. 8 shows an example of a conventional hinge mechanism. Hereinafter, hinge mechanisms for opening and closing will be described. A fixed cam 101 is positioned such that an opening/closing shaft 102 is inserted therein, and by engaging a hole 101-1 of the fixed cam 101 with the shape of the opening/closing shaft 102, the fixed cam 101 is not able to rotate around the opening/closing shaft 102 but it is able to slide in the axial direction of opening/closing shaft 102. A rotating cam 103 is positioned so as not to rotate around the opening/closing shaft with regards to a case 104 by engaging the outside shape of the rotating cam 103 with the inside cross-sectional shape of the case 104, and so as to freely rotate around opening/closing shaft 102 by the circular shape of a hole 103-1 through which the opening/closing shaft 102 is inserted into rotating cam 103. Coil spring 105 is provided between the two fixed cams 101 and the one coil spring drives two pairs of the combination (cam set) of the rotating cam 103 and the fixed cam 101. One end of the opening/closing shaft 102 has a flange 102-1 to prevent disassembly and a mounting head 106 is attached to the other end. The hinge mechanism is constructed by connecting the case 104 to a first member and a head 106 with a second member. Here, the cam which is not able to rotate with regards to the opening/closing shaft is known as a fixed cam, and the cam which is able to rotate is called the rotating cam.
FIG. 9 shows the shape of the cam surface of the fixed cam 101 and the rotating cam 103. The uneven configuration of the cam is usually determined in accordance with the movement range of the device which uses the hinge, and this drawing shows an example of the cam surface of a hinge for a device which opens and closes through a range of between approximately 160° and 170° such as a portable phone. Each cam receives a load from the coil spring 105, and while the protruding region 101-2 of fixed cam 101 and the protruding region 103-2 of the rotating cam 103 are in contact, a sliding torque is generated in the intermediate opening and closing movement range. When the protrusions and recesses of the fixed cam 101 and the rotating cam 103 begin to engage and the slopes 101-4, 103-4 are in contact, the load from the coil spring 105 is converted to a rotational force then a stress for rotation is caused. When the protrusions and recesses of the fixed cam 101 and the rotating cam 103 are completely engaged, the rotational force induced by the load of the spring is reduced, then only the contact force between cam 101 and cam 103 will remain. With standard specifications which have a click generating (absorbing) mechanism, a restricting mechanism for opening and closing is provided between a first and second member, thereby, the designated opening and closing range before the region where rotational forces are eliminated is determined.
The cam shape used for a conventional hinge mechanism is composed such that the cam set will engage in two or more locations of protrusions and recesses as shown in FIG. 9 in order to improve the efficiency of converting the load of the coil spring into a force in the rotational direction at both rotating cam 101 and fixed cam 103. Because the protrusions and recesses are engaged in two or more locations and the protrusions and recesses completely engage two times within an opening and closing angle of 180°, for devices which have an opening and closing range of not less than 180°, a clicking feel is generated by the unnecessary engagement of the cam set across the opening and closing range. As a result, a hinge mechanism in which the first member and second member generate a stress on both ends of the opening and closing range and an excessive click feel is not caused by the engagement of the cams in the opening and closing region cannot be realized. Furthermore, with an actual device, extra room is required in order to generate the stress, therefore, often this mechanism cannot be applied to devices which have an opening and closing range of not more than 180°. Consequently, for devices which require an opening and closing range of approximately 180° or greater, a hinge mechanism which uses a cam cannot be utilized, and this brings a result that a stress cannot be generated at both ends of the opening and closing range or the size of the devices increases by adding a complicated pressing or a lock mechanism.